Interpretive Summary: Pierce’s disease (PD) is an important grapevine abnormality caused by the bacterium Xylella fastidiosa. In California, the bacterium is transmitted by several leafhopper species including the glassy-winged sharpshooter (GWSS), Homalodisca vitripennis. Because infested citrus orchards act as a source of GWSS for adjacent vineyards, a two-year field study was conducted in a citrus orchard to evaluate the influence of plant water stress on GWSS dispersal and movement. Experimental treatments included irrigation at 100%, 80%, and 60% of the crop evapotranspiration (ETc). Movement of H. vitripennis among treatment plots was quantified through a mark and capture technique using protein markers (soy milk, whole milk, and egg white) and yellow sticky traps. Presence of protein markers on insects captured on sticky traps was determined using ELISA. Net dispersal rates showed that GWSS moved constantly among the irrigation treatments and contributed equally to the overall level of population change within treated areas. Trees irrigated at 60% ETc were the least preferred by H. vitripennis adults and yet, about 80% of marked individuals captured in those areas were inflow individuals from the other irrigation treatment areas. Movement towards less preferable host plants indicates that in agricultural landscapes dominated by perennial monocultures, there is a random component to GWSS movement, which may result from the inability of GWSS to make well-informed long-range decisions. Results suggest that GWSS movement from citrus orchards into adjacent vineyards could be a result of random dispersal rather than oriented movement in response to host plant characteristics. Understanding the seasonal dynamics and dispersal behaviors of H. vitripennis and its associated natural enemies under field conditions in California is essential to predicting and managing the continuing threat of grapevine infection by X. fastidiosa.

Technical Abstract:
Homalodisca vitripennis, one of the main vectors of Xylella fastidiosa, is associated with citrus plantings in California, USA. Infested citrus orchards act as a source of vectors to adjacent vineyards where X. fastidiosa causes Pierce’s disease (PD). An analysis of the pattern and rate of movement of H. vitripennis and its egg parasitoid, Gonatocerus ashmeadi, was conducted in a citrus orchard using a protein mark-capture technique to quantify movement and describe the net dispersal rates in the experimental treatment areas. Treatments included irrigation at 100% of the crop evapotranspiration rate (ETc) and deficit-irrigation regimes at 80 and 60% ETc. Sex-specific net dispersal rates showed that H. vitripennis males and females moved constantly among the irrigation treatments and contributed equally to the overall level of population change within treated areas. Trees irrigated at 60% ETc were the least preferred by H. vitripennis adults. Among all protein-marked individuals captured in the 60% ETc treatment areas, ca. 75 and 88% of marked individuals in 2005 and 2006, respectively, were inflow individuals. Movement towards less preferable host plants indicates that in agricultural landscapes dominated by perennial monocultures, there is a random component to H. vitripennis movement, which may result from the inability of H. vitripennis to use plant visual and/or olfactory cues to make well-informed long-range decisions. Results suggest that H. vitripennis movement from citrus orchards into adjacent vineyards and surrounding habitats could be a result of random dispersal rather than oriented movement in response to host plant characteristics.